1. Field of the Invention
The invention relates to a method for driving a power output stage to switch a load connected in series with a MOSFET circuit breaker disposed between the terminals of a voltage source. The invention also relates to a device for implementing the method.
In order to drive inductive loads, power output stages with transistors, in particular MOSFET transistors, are used as the circuit breakers. When the transistors are switched off, the energy of the inductive load has to be dissipated either by zenering the output stage or through a freewheeling diode. In the case of loads with a high inductive energy, the freewheeling diode disposed across the load is used. One disadvantage with such output stages, in particular clocked output stages, is that they cause considerable electromagnetic interference.
One known cause for this is the rapid voltage change at the junction between the load and the circuit breaker. In the case of circuits with MOSFETs, for example, this cause is countered by high-resistance driving of the control terminal by a resistor.
A further cause for the electromagnetic interference that is caused is the rapid change, effected by the switching action, of the current flowing through the circuit breaker.
U.S. Pat. No. 4,661,766 discloses a power output stage for an inductive load with a freewheeling diode, in which the rate of change (current slew rate) of the current flowing through the circuit breaker or through the freewheeling branch is regulated to a predefined value. As a result, however, the switching operation, that is to say the transition from the off state to the on state, or vice versa, is very much delayed.
A similar circuit is disclosed in Published, British Patent Application GB 2 140 996 A, which discloses a method for the regulated switching of a load by use of control signals, in which a circuit breaker, from the start of a control signal, is charged with a charging current at a predefined, high rate of rise until the charging current exceeds a first threshold, then is charged with a charging current at a predefined, low rate of rise until the charging current reaches a second threshold, and is then charged further with a charging current at a predefined, high rate of rise. The rate of rise of the charging current is regulated as a function of the rate of rise of the load current. The discharging operation is carried out in the opposite sequence.
It is accordingly an object of the invention to provide a method and a device for driving a power output stage that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which an inductive load is driven by which the electromagnetic interference can be reduced and the switching delay (delay time) and the power loss can be kept small.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving power output stages. The method includes providing a power output stage for switching a load connected in series with a MOSFET circuit breaker disposed between terminals of a voltage source. The MOSFET circuit breaker is charged with a predefined high charging current until a drain current of the MOSFET circuit breaker exceeds a predefined current threshold from a start of a control signal. The MOSFET circuit breaker is charged with a predefined small charging current until a drain voltage of the MOSFET circuit breaker falls below a predefined voltage threshold, the predefined small charging current being smaller than the predefined high charging current. The MOSFET circuit breaker is charged further, for a predefined charging period, with the predefined high charging current. The MOSFET circuit breaker is charged with a predefined high discharge current until the drain voltage exceeds the predefined voltage threshold after an end of the control signal. The MOSFET circuit breaker is discharged with a predefined small discharge current until the drain current falls below the predefined current threshold, and then, for a predefined discharge period, the MOSFET circuit breaker is discharged with the predefined high discharge current. The predefined small discharge current is smaller than the predefined high discharge current.
The circuit according to the invention provides the advantage that the rate of change of current can be adjusted simply and precisely to be only so steep that no significant interference occurs, but at the same time the switching delay is prolonged only insignificantly.
In accordance with an added mode of the invention, there is the step of determining a maximum rate of rise of the drain current by a value of the predefined small charging current, and a rate of fall of the drain current is determined by a value of the predefined small discharge current.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device containing a positive terminal for a supply voltage and a MOSFET circuit breaker having a gate terminal, a drain and a source. A first constant current source providing a high charging current is disposed between the positive terminal and the gate terminal of the MOSFET circuit breaker. A second constant current source providing a small charging current is disposed in parallel with the first constant current source. A negative terminal for the supply voltage is provided. A third constant current source providing a high discharge current is connected between the gate terminal of the MOSFET circuit breaker and the negative terminal for the supply voltage. A fourth constant current source providing a small discharge current is disposed in parallel with the third constant current source. A control circuit operated on the supply voltage and outputs switching signals to switch the first, the second, the third and the fourth constant current source on and off in dependence on: control signals received by the control circuit, a ratio of a drain voltage of the drain and a predefined voltage threshold, a ratio of a drain current flowing through the MOSFET circuit breaker and a predefined current threshold, and at least one predefined time period.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for driving a power output stage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.